Conventionally, three-phase brushless motors using a signal generated by an inverter as a drive power supply are often used for motors to drive power compressors used for air conditioning apparatuses or the like because such motors have high energy efficiency. In this case, the inverter performs PWM (pulse width modulation) control, applied voltage control and frequency control, and can thereby easily drive the three-phase brushless motor (hereinafter, simply referred to as “motor”) to rotate. Furthermore, the inverter can control rotation of the motor without any position sensors by detecting an induced voltage generated in a coil of the motor. The inverter performs applied voltage control and frequency control of the motor by controlling a duty ratio (ratio of an ON period to a one-cycle period) of a PWM signal for controlling a power device. In addition, the inverter performs motor control by detecting an output voltage or output current of the inverter through a microcontroller and feeding it back to applied voltage control.
As a related art, a technique is disclosed in which PWM-controls an output voltage of an inverter using a microcontroller to thereby drive a motor to rotate (e.g., see PTL 1). According to this technique, when an inverter that performs position sensor-less control detects an induced voltage of the motor and detects and estimates the position of a rotor, it is possible to estimate the position of the rotor with high accuracy by detecting a potential of a virtual neutral point of the motor. This makes it possible to estimate the position of the rotor with high accuracy without using any position sensors to detect the position of the rotor and achieve high efficiency drive of the motor and stable rotation speed.